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Dinosaurs lived on earth 65 million years ago. They lived on the earth much before any human being was born. They ruled the earth for over 165 million years, till 65 million years ago. They were huge monsters, even bigger than elephants. The word "dinosaur" was taken from a Greek word which means terrible lizard. But they weren't exactly lizards. Lizards are one type of reptile and dinosaurs are another type. Not all dinosaurs were giants. Some dinosaurs were no bigger than small dogs. The smallest kind was about the size of a chicken.

No human being has seen a living dinosaur but, scientists have many ways of learning about them. One important way is by studying fossils. Fossils give scientists a vast amount of information about ancient animal and plant life which have become extinct. They also helped them in finding out how long ago the animal or plant lived. As animals and plants die over the years they get covered up and buried into soil. The soft parts of their bodies rot away leaving behind only the hard parts like bones and teeth. These hard parts lose their original mineral composition or structure. Their remaining parts either get recrystallized or replaced by other minerals such as silica and iron pyrites. This is called a fossil. Recrystallization means exchange of one chemical by another within the skeleton which results in a coarser, perhaps heavier structure. Many fossils consist of original hard parts of the animal as well as minerals taken from the surrounding rocks. This process preserves the fossil's structure but increases its hardness and weight.

Plants don't have bones. Only animals have teeth and bones, what happens to plants? Bones and teeth are preserved without much change except that the organic matter in the cavities and tiny spaces in the bone decays, leaving behind the hard, mineral substance of the bone. The spaces left empty by decay get filled with minerals such as lime and silica. Teeth are usually preserved without much change as they are harder than the bones and contain fewer tiny spaces. Aren't some plant fossils only imprints of plants? Jellyfish or other soft bodied animals and plants may be preserved as imprints fossilized over time. However, some of the original leaf substance may be left in the form of a brown or black film of carbon. This process is called carbonization. Other interesting footprints are footprints and trails of certain animals.

Dinosaur tracks have been found in various places. Sometimes, footprints and tracks get wiped out by wind and rain. But in some cases the footprints made on soft clay which later dried up and got covered by layers of fresh sediment and was thus preserved and later turned into sedimentary rock. These fossil footprints have supplied a certain amount of information about how dinosaurs walked and about their size.

Fossilized dinosaur eggs have also been found. The most famous are the dinosaur eggs found in the Gobi Desert in Central Asia. Some dinosaur fossils have been found in other countries also. Based on these, some species have been identified. Some of them are Indosaurus, Megalosaurus, Dravidosaurus, Titanosaurus and Barapasaurus Tagorei. Scientists have been able to identify 350 species of dinosaurs already. There could be many more.

By measuring the age of the rock around a fossil, scientists can tell how old a fossil is. They do this by using several techniques. One such technique is radiocarbon dating which can be used with samples containing remains of living things such as wood. For this they take a small sample of the item to be dated and measure how much of the radioactive isotope, carbon-14, it contains. Isotopes are special forms of an element that have either more or fewer, neutrons than normal atoms of the element. For example, a normal carbon atom contains six neutrons whereas in case of carbon-14 there are eight neutrons instead of the usual six.

Do all organisms have carbon-14? Where does it come from? Plants absorb carbon-14 from the air to produce food by a process called photosynthesis. Plant eating animals acquire this element by eating these plants. When a plant or animal dies its stores of carbon-14 begins to get depleted because like all radioactive elements, carbon-14 loses its atoms over a period of time. Its half-life is 5,568 years. Half-life is a period over which a sample of radioactive element loses about half the number of its atoms. Sample of carbon-14 loses half the number of its atoms in a span of 5,568 years.

The older the fossil the less carbon-14 it contains. But radiocarbon dating can be used to study fossils that are up to about 40,000 years old. For fossils older than this, other method which uses other radioactive elements such as uranium, rubidium, etc. are used. These can accurately determine the age of fossils that are billions of years old.

The geological time scale has been worked out by the paleontologists. The chief divisions in scales are eras, periods and epochs. The eras are the largest of the scale divisions. The major eras were Paleozoic era, Mesozoic era and the Cenozoic era. Among these the Mesozoic era is also called the age of reptiles and is divided into three periods: The Triassic, Jurassic and Cretaceous. The Jurassic period was dominated by dinosaurs. They ruled the land, sea and the air during this period. They were like the kings.

Ð''T-Rex' means Tynnosaurus rex, most ferocious of all the dinosaurs. The name means tyrant-lizard king. It was so scary and so huge. Tynnosaurus stood nearly 3 meters high at the hips and grew about 13 meters long. Their head measured up to 15 cm long. They had short forelimbs compared to their bodies which were almost useless. Each forelimb had only two fingers. Tyrannosaurus ruled the land near the end of the Age of Reptiles. The long tail was muscular which they carried straight out behind them for balance. But, for it the T. rex could not have walked upright on its two hind legs. Allosaurus is also a dinosaur.

The Brachiosaurus was such a darling. It didn't harm the children at all. Brachiosaurus was the largest known dinosaur. It stood 13 meter tall or more and weighed up to 77 metric tons. They had long forelimbs and walked on four legs like an elephant. Their front legs were longer than the back ones. They stood like giraffes, with the back sloping down toward the tail.

Triceratops was the most famous horned dinosaur. It had three horns and a frill. It was not a flesh-eater. It was a plant eater. The horns and frill most likely gave protection from predators. When threatened these creatures could fight back with their horns, which could be more than a meter long. The frills anchored strong muscles which made their jaws powerful cutting blades. It also protected the Triceratops neck, the point at which most predators attack. Their mouth was



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